Current aircraft production pace may be adversely affected by unavailability of sufficiently durable, reliable, and uniform fasteners for attaching various parts of the aircraft together. Fastener production throughput may be limited by the machines, operations, and subsequent quality assurance efforts that must be configured and used for a fastener production run.
For example, production and testing efforts are taken in traditional fastener manufacturing to ensure proper grain boundary control in the bulk metal. Attaining the desired grain boundaries for a particular fastener design may involve multiple machining and inspection steps, each of which adds time and cost to the manufacturing process. The grain boundary considerations, particularly the grain boundaries around load-bearing and contacting surfaces of fasteners, may be of concern to one of ordinary skill in the art at least because grain boundaries offer sites for propagation of corrosion, defects, stress concentrations, and fatigue.
There are currently two main bolt-type fasteners used in aircraft use environments: (1) threaded, and (2) swaged systems. Both of these types of bolt-type fasteners rely on a relatively large amount of surface contact (via friction or mechanical lock, respectively, on a thread) to provide sufficient holding torque for a particular use application. Clamp-up of a threaded fastener system may be susceptible to corruption/error due to contamination, improper part geometry (e.g., slight nonconformances, misalignment during installation), installation methods, and the like. Clamp-up of a swaged fastener relies upon the ability of relatively soft materials to be effectively “crushed” into place, in addition to the error sources present with threaded fasteners.
Because of variable clamp-up results in these known systems, designers cannot effectively rely upon fasteners being useful in the field up to the “perfect condition” fastening strengths theoretically available, and therefore fastened assemblies are often conservatively designed, even overdesigned. As a result, there are generally more and/or larger traditional threaded and/or swaged fasteners provided than would be needed with more reliable clamp-up results, and the final design is more expensive and heavier than they would need to be if the clamp-up forces were more predictable and regulated.
In addition, traditional threaded or swaged fasteners generally have “revolved” (i.e., rotationally symmetrical) shapes, due to the relative ease of machining those shapes as compared to non-revolved shapes). Particularly when exposed to vibration or shock, revolved-shaped fasteners tend to rotate, which can lead to degradation of coatings and paint applications and increased wear. This “creep” or “loosening” can cause a swage collar or threaded nut to eventually work its way loose from the bolt, causing potential failure of the fastener system.